Covalent Bonding and Molecular Compounds Chemical Bonding

Covalent Bonding and Molecular Compounds

Chemical Bonding

Covalent Bonding and Molecular CompoundsObjectives

Define molecule and molecular formula

Explain the relationships among potential energy, distance between approaching atoms, bond length, and bond energy

State the octet rule

List the six basic steps used in writing Lewis structures

Covalent Bonding and Molecular CompoundsObjectives

Explain how to determine Lewis structures for molecules containing single bonds, multiple bonds, or both

Explain why scientists use resonance structures to represent some molecules

Covalent Bonding and Molecular CompoundsMolecular Compound

A molecule is a neutral group of atoms that are held together by covalent bonds

A chemical compound whose simplest units are molecules is called a molecular compound

Covalent Bonding and Molecular CompoundsMolecular Compound

The composition of a compound is given by its chemical formula

A chemical formula indicates the relative numbers of atoms of each kind in a chemical compound by using atomic symbols and numerical subscripts

A molecular formula shows the types and numbers of atoms combined in a single molecule of a molecular compound

Covalent Bonding and Molecular CompoundsChemical Formula

Covalent Bonding and Molecular CompoundsMonatomic, Diatomic, and Polyatomic Molecules

Covalent Bonding and Molecular CompoundsFormation of Covalent Bonds

Most atoms have lower potential energy when they are bonded to other atoms than they have as they are independent particles

The figure below shows potential energy changes during the formation of a hydrogen-hydrogen bond


The electron of one atom and proton of the other atom attract one another

The two nuclei and two electrons repel each other


These two forces cancel out to form a covalent bond at a length where the potential energy is at a minimum

Covalent Bonding and Molecular CompoundsCharacteristics of Covalent Bonds

The distance between two bonded atoms at their minimum potential energy (the average distance between two bonded atoms) is the bond length

In forming a covalent bond, atoms release energy. The same amount of energy must be added to separate the bonded atoms

Bond energy is the energy required to break a chemical bond and form neutral isolated atoms

Covalent Bonding and Molecular CompoundsBond Energy


When two atoms form a covalent bond, their shared electrons form overlapping orbitals, This achieves a noble-gas configuration

The bonding of two hydrogen atoms allows each atom to have the stable electron configuration of helium, 1s2


Atoms can fill their outermost s and p orbitals by sharing electrons through covalent bonding

Such bond formation follows the octet rule: chemical compounds tend to form so that each atom, by gaining, losing, or sharing electrons, has an octet of electrons in its highest energy level

Covalent Bonding and Molecular CompoundsThe Octet Rule


Exceptions to the octet rule include those for atoms that cannot fit eight electrons, and for those that can fit more than eight electrons, into their outermost orbital

Hydrogen forms bonds in which it is surrounded by only two electrons

Boron has just three valence electrons, so it tends to form bonds in which it is surrounded by six electrons

Main-group elements in Periods 3 and up can form bonds with expanded valence, involving more than eight electrons

Covalent Bonding and Molecular CompoundsElectron-Dot Notation

To keep track of valence electrons, it is helpful to use electron-dot notation

Electron-dot notation is an electron-configuration notation in which only the valence electrons of an atom of a particular element are shown, indicated by dots placed around the element’s symbol. The inner-shell electrons are not shown




Sample Problem

Write the electron-dot notation for hydrogen

Write the electron-dot notation for nitrogen


Sample Problem Solution

A hydrogen atom has only one occupied energy level, the n = 1 level, which contains a single electron

The group notation for nitrogen’s family of elements is ns2np3, nitrogen has five valence electrons

H

N

Covalent Bonding and Molecular CompoundsLewis Structures

Electron-dot notation can also be used to represent molecules

The pair of dots between the two symbols represents the shared electron pair of the hydrogen-hydrogen covalent bond

For a molecule of fluorine, F2, the electron-dot notations of two fluorine atoms are combined

H H

F F


The pair of dots between the two symbols represents the shared pair of a covalent bond

In addition, each fluorine atom is surrounded by three pairs of electrons that are not shared in bonds

An unshared pair, also called a lone pair, is a pair of electrons that is not involved in bonding and that belongs exclusively to one atom

F F

F F


The pair of dots representing a shared pair of electrons in a covalent bond is often replaced by a long dash

A structural formula indicates the kind, number, and arrangement, and bonds but not the unshared pairs of the atoms in a molecule

H H F F

F–F H–Cl

Covalent Bonding and Molecular CompoundsStructural Formula


The Lewis structures and the structural formulas for many molecules can be drawn if one knows the composition of the molecule and which atoms are bonded to each other

A single covalent bond, or single bond, is a covalent bond in which one pair of electrons is shared between two atoms


Sample Problem

Draw the Lewis structure of iodomethane, CH3I



Determine the type and number of atoms in the molecule. The formula shows one carbon atom, one iodine atom, and three hydrogen atoms

Write the electron-dot notation for each type of atom in the molecule. Carbon is from Group 14 and has four valence is from Group 17 and has seven valence electrons. Hydrogen has one valence electron

C I H



Determine the total number of valence electrons available in the atoms to be combined

C 1 × 4e– = 4e–

I 1 × 7e– = 7e–

3H 3 × 1e– = 3e–

14e–



If carbon is present, it is the central atom. Otherwise, the least-electronegative atom is central. Hydrogen, is never central

Add unshared pairs of electrons to each nonmetal atom (except hydrogen) such that each is surrounded by eight electrons

CHH

HI

HC IH

Hor H C

H

I

H

Covalent Bonding and Molecular CompoundsMultiple Covalent Bonds

A double covalent bond, or simply a double bond, is a covalent bond in which two pairs of electrons are shared between two atoms

Double bonds are often found in molecules containing carbon, nitrogen, and oxygen

A double bond is shown either by two side-by-side pairs of dots or by two parallel dashes

HC

H HC

Hor

HC

HC

H

H


A triple covalent bond, or simply a triple bond, is a covalent bond in which three pairs of electrons are shared between two atoms

diatomic nitrogen

ethyne, C2H2

N N or N N

C C or C CH H H H


Double and triple bonds are referred to as multiple bonds, or multiple covalent bonds

In general, double bonds have greater bond energies and are shorter than single bonds

Triple bonds are even stronger and shorter than double bonds

When writing Lewis structures for molecules that contain carbon, nitrogen, or oxygen, remember that multiple bonds between pairs of these atoms are possible


Covalent Bonding and Molecular CompoundsDrawing Lewis Structures

Covalent Bonding and Molecular CompoundsDrawing Lewis Structures


Sample Problem

Draw the Lewis structure for methanal, CH2O, which is also known as formaldehyde



Determine the number of atoms of each element present in the molecule. The formula shows one carbon atom, two hydrogen atoms, and one oxygen atom

Write the electron-dot notation for each type of atom. Carbon is from Group 14 and has four valence electrons. Oxygen, which is in Group 16, has six valence electrons. Hydrogen has only one valence electron

C HO



Determine the total number of valence electrons available in the atoms to be combined

C 1 × 4e– = 4e–

O 1 × 6e– = 6e–

2H 2 × 1e– = 2e–

12e–



Arrange the atoms to form a skeleton structure for the molecule. Connect the atoms by electron-pair bonds

Add unshared pairs of electrons to each nonmetal atom (except hydrogen) such that each is surrounded by eight electrons

CCH C OH

H C OH



Count the electrons in the Lewis structure to be sure that the number of valence electrons used equals the number available. The structure has 14 electrons. The structure has two valence electrons too many

Subtract one or more lone pairs until the total number of valence electrons is correct. Move one or more lone electron pairs to existing bonds until the outer shells of all atoms are completely filled



Subtract the lone pair of electrons from the carbon atom. Move one lone pair of electrons from the oxygen to the bond between carbon and oxygen to form a double bond

H C OH

H C OH

or

Documents

Covalent Bonding and Molecular Compounds Chemical Bonding